This invention relates to a heat-insulated structural section assembly for use in manufacturing window and door frames which consists of two structural metal sections transversely connected to each other by a pair of plastic heat-insulating bars with which they define an enclosed air space. Each structural metal section has flanges integrally formed therewith, which define with the web of the section longitudinally extending grooves in which said heat-insulating bars are adapted to be clamped by said flanges.